Add can_send_version() to RpcProxy.

Add a helper method to the RpcProxy class.  This is a little nicer to
use for checking to see if a given message is copmatible with the set
version cap.

Change-Id: Ic44d76f4181351dff367f2d1181a3d508a11db78
Merge "Add can_send_version() to RpcProxy."